This invention relates to a receiver for a digital transmission system, and more particularly to a receiver for a superpose modulated signal for use in digital transmission systems. That is, the transmitted signal is a superpose modulated signal.
Binary data signals are usually modulated onto a single carrier or onto quadrature carrier signals for transmission to a receiver. The last stage prior to transmission is usually high-powered amplification. Efficient high-powered amplifier operation requires that the amplifier be operated in its saturation mode, which results in non-linearity of the output. Therefore, employing a high-powered amplifier results in the creation of numerous sidebands, which usually causes inter-channel interference. Accordingly, modulation methods are required which are capable of solving this problem, with the efficient use of power and bandwidth in transmission.
Modulation methods satisfying with above requirement are disclosed in U.S. Pat. No. 4,399,724 by Kamilo Feher and U.S. Pat. No. 4,644,565 by Jong-soo Seo and Kamilo Feher. In both of these patents, a bit in pulse form (such as non-return-to-zero) is transformed into output signal which corresponds one of four specified signals based on the previous bit and the present bit to be transmitted. In the former patent, the four specified signals are Acos(.pi.t/T), -A, -Acos(.pi.t/T) and A, where A is an amplitude parameter and T is the bit duration. Meanwhile, in the latter patent, the four specified signals are -A-(1-A)cos(2.pi.t/T), -cos(.pi.t/T), cos(.pi.t/T) and A+(1-A)cos(2.pi.t/T), where likewise A is an amplitude parameter and T is the bit duration. These modulated signals have continuity even at the bit transition position and have no .jitter and no inter-symbol interference. More particularly, the latter modulation method is able to control the bandwidth of the transmitted signal to suit the transmission system, by use of the amplitude parameter A. The signal thus-modulated is called a superpose modulated signal.
Hereinafter, conventional receivers for demodulating the superpose modulated signal which may include noise, will be explained.
Among conventional receivers, there are a matched filter receiver and optimum receiver. The matched filter receiver includes a filter whose transfer characteristic matches that of another filter in the transmitter, resulting in reducing the noise signal and maximizing the original signal in the received signal. The transmitted signals, however, have very complicated components as described above, thereby the implementation of the matched filter receiver being difficult. Accordingly, a physical filter receiver can be used instead of the matched filter receiver. The physical filter receiver is, for example, a Butterworth filter having a 3 dB frequency (or half-power point) which is the half the bit frequency.
In the convention physical filter, however, changing the bit rate of the transmitter changes its half-power frequency (3 dB point). Here, determining the half-power frequency of the physical filter in the receiver is critical, because an improperly set half-power point results in the degradation of the bit energy-to-noise density ratio and/or attenuating the original signal components, thereby increasing the error rate. For improvement of this problem, a filter is disclosed in which one of a plurality of half-power frequencies is selected at any given time and in accordance with the bit rate of the transmitter. However, this filter requires an excessive amount of hardware whose implementation increases costs accordingly. The conventional physical filter must also change its characteristics in accordance with an amplitude parameter or superposed parameter A.
Therefore, in a conventional transmission system, the receiver's filter itself is generally changed, according to the bit rate of the transmitter.
Another conventional receiver, the optimum receiver, can reduce the error rate in demodulation. In the optimum receiver, correlation pulses are generated which correspond to the baseband signals of the transmitter, and then the received signal is compared with the correlation pulses for demodulation. The optimum receiver can also change its transfer characteristic by adjusting the periods of the correlation pulses, in the case of a changed bit rate in the transmitter, for matching itself with the transmitter.
Here, the superpose modulated signal includes a plurality of baseband signals, so that the implementation of the optimum receiver related to a superpose modulate signal requires a plurality of generators for respectively generating baseband signals and a plurality of detectors for respectively producing observation signals. The optimum receiver must also include a selector for selecting one of the observation signals. Therefore, the implementation an optimum receiver is complex.
For reducing such complexity, a sub-optimum receiver is disclosed, which is constituted based on fewer baseband signals than those of the optimum receiver, thereby simplifying its structure though increasing the probability of error. For example, the optimum receiver for minimum shift keying (MSK) can be used for demodulating the superpose quadrature modulated signal as a sub-optimum receiver. Here, the MSK optimum receiver produces a plurality of correlation pulses with respect to each MSK baseband signal, and the baseband signals for MSK are similar to those for a superpose quadrature modulated signal. In more detail, the smaller the amplitude parameter A is, the greater the similarity between the baseband signals for superpose quadrature modulated signal and those for MSK is, so as to reduce the mismatching degree between transmitter and receiver. That is to say, the demodulation for a superpose quadrature modulated signal with MSK optimum receiver may be performed with a negligible amount of degradation in the probability of error. Here, the smaller the amplitude parameter A is, the smaller the probability of error is. However, the occupied bandwidth of the superpose quadrature modulated signal increases according to the reduction of the amplitude parameter A, based on the characteristics of the superpose quadrature modulated signal, thereby decreasing bandwidth efficiency. Here, it should be noted that occupied bandwidth for MSK is wider than that for a superpose quadrature modulated signal within the amplitude parameter (0.5.ltoreq.A.ltoreq.1.0) and that MSK has side lobes which are appreciable. Accordingly, the sub-optimum receiver for superpose quadrature modulated signals, which is constituted based on the baseband signals for MSK, has transfer characteristics consistent with that of an MSK transmitter. Therefore, the occupied bandwidth of the sub-optimum receiver is increased according to the MSK-transmitted bandwidth, such that it receives more adjacent channel signals, which increases the probability of error for a multi-channel transmission system.